(1) Field of the Invention
The present invention relates to a semiconductor chip element to which ultrasonic bonding is applied, a semiconductor chip element mounting structure and a semiconductor chip element mounting device and mounting method.
(2) Description of the Related Art
FIG. 1A shows a conventional semiconductor chip element 10 to be ultrasonically bonded. The semiconductor chip element 10 comprises a silicon chip 11 on a bottom surface 11a of which an integrated circuit 12 is formed and electrodes 13 made of aluminum formed at a periphery of the bottom surface 11a, on top of which are formed signal stud bumps 14, power stud bumps 15, ground stud bumps 16 (not shown in the diagram) and dummy stud bumps (not shown in the diagram). The signal stud bumps 14, power stud bumps 15, ground stud bumps 16 (not shown in the diagram) and dummy stud bumps (not shown in the diagram) are all made from 99.99% pure gold in order to lower the electrical resistance of the connecting portion connecting to a circuit board of the semiconductor chip element.
It should be noted that the stud bumps 14, 15 are formed by using a wire bonding device from the nozzle of which a gold wire is extended, and moving the wire bonding device in generally the same pattern as when performing wire bonding, with this difference: Whereas in wire bonding the wire is extended continuously from bond to bond, in forming the stud bumps 14, 15 the wire is cut at each bump, giving the bumps the substantially onion-domed shape shown in the diagrams.
As shown in FIG. 1B, the semiconductor chip element 10 is mounted in a face down position on a circuit board 30 that is itself fixedly mounted on top of a table 20, in such a way that each stud bump 14, 15 is aligned with a corresponding gold electrode 31. Under room-temperature conditions, the semiconductor chip element 10 is pressed with a tool 21 and at the same time ultrasonic vibrations are applied for a period of, for example, several seconds, with the surfaces of the stud bumps 14, 15 and the electrodes 31 (both made of gold) rubbed against each other so as to ultrasonically bond the stud bumps 14, 15 and the electrodes 31.
With the semiconductor chip element 10 in the state described above, using a device (not shown in the diagrams) an underfill 41 is injected into a gap 40 between the underside of the semiconductor chip element 10 and the circuit board 30 as shown in FIG. 1C, after which the underfill undergoes thermosetting.
As shown in FIG. 1C, the semiconductor chip element 10 is mounted on the circuit board 30 in a state in which the stud bumps 14, 15 are ultrasonically bonded to the electrodes 31 and the gap 40 is filled with the underfill 41 which is an epoxy resin. The underfill 41 strengthens the attachment of the semiconductor chip element 10 to the circuit board 30 and protects the integrated circuit 12.
It should be noted that the 99.99% pure gold only has a hardness of 75 Hv on the Vickers hardness scale, that is, is soft. As a result, it sometimes happens that the stud bumps 14, 15 suffer substantial deformation when ultrasonically bonded to the electrodes, in which case a height A of the gap 40 described above narrows sharply, to approximately 10 μm. When the gap 40 narrows, not enough of the underfill 41 enters the gap 40 to fill the gap 40 completely and so an unfilled space indicated in FIG. 1C by reference numeral 42 is left.
It will be appreciated that the presence of the unfilled space 42 reduces the effectiveness of the underfill 41 and hence reduces the strength of the attachment or mounting of the semiconductor chip element 10 on the circuit board 30, and additionally, fails to fully protect the integrated circuit 12.
It is possible to maintain the gap 40 at a height A greater than 10 μm by raising a height h of the stud bumps 14, 15. However, since the stud bumps 14, 15 are formed in much the same way that wire bonding is performed, if the height h is increased then a diameter d of the stud bumps 14, 15 also increases, yet at the same time the pitch of the stud bump alignment imposes limitations on the diameter of the stud bumps 14, 15. As a result, there is a practical limit to the extent to which the height h of the stud bumps 14, 15 can be increased.